1. Field of the Invention
The invention relates to a photoelectric control unit for a lighting circuit as a street lighting circuit.
2. Description of Prior Art
In general, street lamps are operated by photoelectric control units comprising a photoelectric sensor and a load switching means. The control unit turns on the lamp when the intensity of incident light falling on the photoelectric sensor is below a pre-set value and turns off the lamp when the intensity of the light is above another pre-set value. Such control units are generally connected to a lamp through connector pins. Heat is generated in these connector pins directly by the current carried by the pins and indirectly by heat from the lamp. This heat may cause mal-functioning of the control unit which greatly reduces the useful life of such units. One such unit is described in British Patent Specification No. 1,110,263.
The controllers for lighting control units which are presently available are either electro-mechanical, electronic or a combination of both. Electro-mechanical units switch power to a load by opening or closing a pair of heavy duty electrical contacts. Generally the line voltage has a peak value of over 310 volts which causes arcing across the contacts as they open and close. The arcing in turn causes pitting of the contact surfaces resulting in a short useful life-span for the controller.
A further problem with conventional controllers is that in time, the light-level threshold can drift outside the specified limits due to drift of the threshold reference value and changes in the characteristics of the light detecting sensor which is normally a light dependent resistor or LDR. Conventioanl units employ a thermal switching technique where the LDR controls the current through, and thus, the temperature of a bi-metallic strip. The threshold point is determined by the force required from the thermal strip to change over spring loaded electrical contacts. The characteristics of the spring load as well as those of the bimetallic strip change with time and this causes the accuracy of the unit to drift outside the acceptable limits. Also, the LDR is required to conduct large currents to heat the bi-metallic strip and the resulting self-heating effect causes the LDR to change its characteristics over a period. The LDR units commonly used in existing controllers, gradually break down allowing moisture and impurities to effect the selenium cell. This combined with temperature rise in the device contributes to further drift of the light level threshold.